Auth transports add credentials after cross-origin redirects

## Summary

`BasicAuthTransport` and `UnauthenticatedRateLimitedTransport` add credentials in `RoundTrip` for each outgoing request. When Go's `net/http` client follows a redirect to a different origin, the redirected request can pass through the same transport and receive those credentials again.

## Expected behavior

Credentials added by these transports should only be sent to the original origin. Redirected requests that cross origin should continue without these transport-managed credentials.

## Reproduction

This can be reproduced with two `httptest` servers: one server returns a redirect to a second server, and the second server records whether the redirected request contains the transport-added authorization headers.

## Proposed fix

PR #4364 avoids adding these credentials on cross-origin redirect requests and includes regression coverage for both affected transports.

Inizia con BasicAuthTransport e UnauthenticatedRateLimitedTransport, in particolare con i relativi metodi RoundTrip, e riproduci il comportamento con due server httptest come descritto. Il lavoro è completato quando esiste una copertura di regressione per entrambi i transport e le richieste reindirizzate a un’origine diversa non contengono i relativi header di autorizzazione aggiunti dal transport.

Scritto dal modello di indicizzazione a partire dal testo della issue.